Reformat use declarations.

The previous commit updated `rustfmt.toml` appropriately. This commit is
the outcome of running `x fmt --all` with the new formatting options.

compiler/rustc_codegen_gcc/build_system/src/main.rs

@@ -1,5 +1,4 @@
-use std::env;
-use std::process;
+use std::{env, process};
 
 mod build;
 mod clean;